of the Orion nebula.
How can I avoid that ?
orion3.jpg by Radu Coman, on Flickr
Sky Watcher 750/150 with the 50D in prime focus, 28 exposures of 45 seconds with darks flats and bias frames (I think that 45 seconds are too much for the EQ3 mount) stacked in DSS curves and levels adjusted in PS.
Bob how do you get those pinpoint stars ?

Bob had previously posted a technique for separating the stars from the rest, allowing you to manipulate them independently for recombination. I still haven't given that a serious try yet... in this case, I don't mind that the stars have a size to them.

I love the detail you got here... is this with an unmodified 50D? Any extra filters? Are you in an area of low light pollution?

As for the blown core... my only thought is to do some lesser exposures for that region, then recombine them later e.g. by HDR like methods.

Hi popo,
Yes it’s the unmodified 50D. No filters. I drove 1 ½ hours to the location, on the heights near the Dead sea. I’m doing this with folks from the astronomy association once a month, obviously when is a new moon. The visibility was good, but not so good as other times, and the only major light pollution is from Jerusalem, about 30 km away. North east and it’s felt up to about 15 to 20 degrees above the horizon. So it was good but not great. The Milky wai is visible to the naked eye. The wind was more problematic , about 11 km/h, and that’s too much for the EQ3 mount.
About the blown core. If I make some frames with let’s say 1 ½ stops less ISO and stack them in DSS with the higher sensitivity ones, would solve the problem ?

The procedure needs practice and a willingness to experiment but I'm already at the stage where I'd be lost without it. Yes, you can separate the stars from an image and allow separate workflows for stars and nebulosity but, provided you have already aligned the frames, it is just as easy to combine stars from a totally different set of subs to those used to capture the nebulosity. I've yet to tackle the Orion nebula as I'm still waiting for my TEC 140 'scope (4 months late now!) but I do know that you will have to take subs with a very wide range of exposures and combine them during post-processing. I imagine that there will be plenty of tutorials out there on how to use PhotoShop layers to do the job. At the end of the day if you can remove the stars from all the nebulosity frames you are left with essentially a standard HDR situation with bracketed frames. Once the nebula image is to your liking then add back the stars that look most appropriate.

All theory on my part however, even though it is how I intend to work when I get the chance, so please take the above with a large pinch of salt.

Hi Bob and popo,
Thanks for the explanations and the tips. I’ll surely have another go at it in about three weeks. I wanted to go the last Thursday but, alas, clouds came in unexpectedly (in spite of the forecast) so everybody stayed home.
My stars are egg shaped. Is this coma aberration or tracking error? I think that coma should be evident towards the edge of the frame ( what you see is not cropped) and not in the centre of it. But I’m not sure. Investing now in a better mount is not possible, so I wonder if a coma corrector will improve the situation

Your image responds well to the star removal technique. The stars can be sharpened up and, when working with the "no stars" part PhotoShop's Shadows/Highlights adjustment can be applied to recover more of the faint stuff - even from the JPEG there's more there than is being easily seen. A little judicious noise removal in the fainter parts of the nebulosity helps mitigate the effect of the Shadows/Highlights stretch (Topaz Labs DeNoise PhotoShop plug-in allows you to tune noise reduction in that way) and the stars can then be added back. Nothing to be done about the blown core, of course. If you aren't imaging for a little while give it a try.

Bob.

P.S. I'm assuming you have PhotoShop, of course. Apologies if you haven't.

Tried out PixInsight ond the same data. Almost. No flats, darks nor bias, I don't know how to do that yet. The Trapezium is stil blown out but the core is much better. Rain and clouds prevented me to go out and grab some more on the new moon.
PixInsight rawks M42_V3.jpg by Radu Coman, on Flickr

Hi mbrandste, thanks for the compliment. I know it’s still a far cry from what it should be, but you know , It’s great fun.
All the time effort, time, and money invested in one shot gives you lots of satisfaction combined with lots of frustration. Reminds me what I felt the first times, as a 10 years old, when I was developing my first photographic papers and saw in the dim red light the image magically revealing itself.